We may finally mention, without describing'. any of the many types, the machines for pro cessing (including fixing and washing) cinc`; film, film in long strips for aerial photography, and roll-films used in cameras for amateurs.
264. Materials for Tanks, Hangers, and Dishes. Owing to its absolute impermeability and resistance to nearly all chemical reagents, glass is certainly the best material for the manufacture of tanks and dishes, at least in the sizes required by the amateur. After proper cleaning, a glass dish can retain none of the products it previously held, and can therefore be used alternately for very different operations without risk of con tamination. Transparent glass dishes are some times used in order to be able to illuminate the negatives from below in the course of development or fixing in order to follow the progress of the operation without touching the plates.
Porcelain (white translucent ware) has the same properties, but is no longer used for dishes and tanks. Dishes sometimes described as porcelain " are nearly always made of some variety of earthenware (opaque porous ware) with the surface protected by a glaze, which is fragile and easily chips off. This glaze readily cracks, allowing the products to penetrate into the porous substance, whence they can only be partially removed by washing, or even by the action of powerful reagents. Not only should an earthenware dish be kept as far as possible for the same bath, but one should avoid leaving the solution in it longer than is necessary for the operation ; for if a solution of a salt is left in such a dish it tends to crystallize out in the earthenware, breaking up the glaze and some times even the earthenware itself. This does not apply, however, to the special stoneware made for the chemical industry and used for the manufacture of vertical tanks for handling numbers of roll films at a time.
Brief reference may be made to dishes of japanned card, also to tanks and dishes of cellu loid or of moulded composition. These nearly always split or become deformed after using a few times." Slate is excellently adapted for the construc tion of dishes of large size, being strong and hav ing perfect resistance to almost all chemical reagents. 2 At the same time, it is very necessary
to protect the edges of the tanks used for fixing to prevent the hypo from penetrating into the material along the planes of cleavage, and separating the layers when it crystallizes. This, however, only occurs after very long use.
Tanks of cement are useful for washing ; they can also be used for development in dilute baths or baths only slightly alkaline. They are attacked by fixing baths, particularly acid fixers, as well as by all acids in very weak solution.
Dishes of wood lined with gutta-percha about of an inch thick are very satisfactory in large sizes by reason of their lightness and strength. They are not much use, however, in hot climates, where the gutta-percha becomes very soft. If left dry for a great length of time these dishes are liable to crack, but it is easy to repair them with a hot iron. Wooden dishes may be made impermeable with molten paraffin wax applied very hot with a brush and smoothed on the surface with a hot iron ; also with several coatings of asphalt varnish, allowed to dry with free exposure to the air before using. 3 Vessels of enamelled cast iron made by manu facturers for the chemical industry resist the usual photographic reagents well. The iron does not become deformed, and the enamel, having the same coefficient of expansion as the iron, does not tend to crack. Unfortunately, it is not the same with dishes of enamelled sheet iron, or enamelled steel, of which the coating is often cracked before delivery.' Once the metal underneath is bare its corrosion is rapid, and the enamel soon comes off in flakes on both sides of the cracks. Also, bad quality enamels often used in this branch of manufacture are super ficially attacked by the alkali of developers, and are then by no means so easy to clean as when the surface is polished.
Ebonite-covered steel, which is sometimes used for medium-sized tanks, is not open to the same objections, as ebonite is sufficiently elastic not to crack in case of slight temporary or permanent distortion of the metal.